Depends on the web page - you can click the green 'make window the right size' button in the upper left, which will maximize or attempt to fit the window to the content, depending on what's in it, but if the web page displayed is designed to be too large for your screen, that will only help so much.

Similarly, you can choose your text size larger or smaller to adjust how it displays.

However, if there are graphics or other large items on the page being displayed, Safari, like all other browsers, will do its best to show what it can, and give you scroll arrows for the rest.

I don't know of any browser that has a global 'fit contents to window' option, though it's not a bad idea.